Method and apparatus for multiplex telegraphy



Nov. 24, 1959 R. HEIDESTER 2,914,611

METHOD AND APPARATUS FOR MULTIPLEX TELEGRAPHY Filed Oct. 25, 1955 2 Sheets-Sheet 2 INVENTOR RUDOLF HEIDESTER BY QM PATENT AGENT United States Patent ice METHOD AND APPARATUS FOR MULTIPLEX TELEGRAPHY Rudolf Heidester, Ulm (Danube), Germany, assignor to Telefunken G.m.b.H., Berlin, Germany 7 Application October 25, 1955, Serial No. 542,724 Claims priority, application Germany October 28, 1954 Claims. (Cl. 17851) The invention relates to multiplex telegraphy, i.e. to a method and apparatus of receiving two telegraphic signals transmitted concurrently by frequency-shift mark and space-transmission employing four degrees of shift symmetrically arranged about a center channel frequency.

It has been known in the transmission of telegraphic intelligence, for example in the radio transmission of such intelligence, to use frequency-shift methods. In these methods the mark and space signals are produced by shifting from one frequency to another. Altogether four different frequencies are required to transmit two separate telegraphic messages by a single transmitter, whereby only a single frequency is transmitted at any particular instant. Several of such methods have been known. In the Twinplex system, also known as the Diplex system, there is assigned to the one channel a certain frequency shift and to the other half of such frequency shift.

The diagram of Fig. 1 shows schematically how the four frequencies are assigned to the two channels in the case of the Twinplex system. The four lines 11, f f and 1, represent the four frequencies arranged symmetrically with respect to a central channel frequency indicated by a dash-dot line. The channel A is characterized in that during the transmission of the space signal T the frequency (f or f lies, for example, on the left side of the central channel frequency, and in case of the transmission of the mark signal Z, the frequency 3 or f.,) lies on the right side of the central channel frequency. Whether the frequency lies at f f f;, or depends upon the other channel signal B at any particular instant. The space signal T for the channel B is given by the frequency f or and the mark signal Z by the frequencies or 12,. Whether the frequency f or 2, occurs depends upon the signal in channel A. In the Diplex system, the assignment of the four frequencies to the channel A is the same as in the Twinplex system. However, the frequencies in case of the channel signal B are assigned as schematically indicated in the diagram of Fig. 2.

In case of the Twinplex system according to Fig. 1, the whole signal at the transmitter side is composed of the two channel signals A and B in such a manner that the transmitter for the channel signal A supplies a current of twice the value (+2, 2 in the diagram Fig. 3) as the transmitter for the channel signal B (+1, +1 in the diagram Fig. 3) to the common load resistance R of the direct current circuits. As a result of this, four different voltages (+3, +1, 1, -3) are obtained at the common load resistance R as a function of the instantaneous signals of the channels A and B, said voltages controlling the frequencies of the transmitter, for example, by means of a reactance tube. In this case, the same relation as mentioned in the foregoing with reference to Fig. 1 exists between the voltages and the signals in the two channels A and B. It has been known heretofore to separate the received four frequencies from one another, preferably, in the low frequency circuits by means of filters. It has also been known to save filters by using members with frequency-dependent phase rotation, as well. as phase modulation.

These known systems have the disadvantage that it is possible only to receive signals transmitted with a certain separation between the four frequencies, said separation generally amounting to 400 cycles. More recently transmitters have been known in which a larger or smaller frequency separation is used.

It has been known to omit filters when separating two channels, and to obtain four different D.C. voltages cor responding to four signal frequencies by frequency demodulation. A discriminator with two oppositely polarized sets of rectifiers serves as the frequency demodulator, so that two intersecting demodulation characteristics are obtained. Thus, actually; there are two oppositely acting discriminators with common oscillator circuits. The signal of the channel A is determined by the polarity of the output voltage of the one discriminator. Three biased tubes acting as switches are employed to obtain the channel B. One of the tubes will be opened by one discriminator in response to the frequency and the second tube will be opened by the other discriminator in response to the frequency f;,. In order to prevent the second tube from being opened by the frequency 1, the output voltage of the second discriminator is rendered ineffective during the occurrence of said frequency f by means of a third switching tube controlled by one rectifier of the second discriminator. In this way, filters are avoided; however, the biasing voltages of the three switching tubes have to be separately adjusted in case of a change in the shift between the four frequencies. i

The invention shows simpler circuit arrangements which, in addition, make it possible to adapt the receiver to a changed frequency shift solely by adjusting a single bias voltage.

The invention relating to the reception of two channels, transmitted from the same transmitter by means of frequency shifts, provides that one discriminator having a single demodulation curve serves for frequency demodulation and also supplies a direct current voltage to a symmetrical amplitude clipping device in order to obtain a signal for channel A for which the polarity of the output voltage of the discriminator determines the space or mark signal. The clipping level of this symmetrical amplitude clipper device is equal to or smaller than the direct current voltages which correspond to the two frequencies nearest the central channel frequency. The other channel B of the output voltage of the discriminator is demodulated, depending upon the assignment of the four channel frequencies to the mark and space signals of this channel B, so that only the latter channel is produced. In case of the Twinplex system, the demodulation is carried out in such a manner that a certain portion of the output voltage of the discriminator is added to' the output voltage of the amplitude clipper, while in the Diplex system, the demodulation takes place in an other manner, as will be explained below.

These and other objects and advantageous features of this invention will be apparent from the following detailed description and drawings, appended thereto, wherein merely for the purposes of disclosure non-limitative embodiments of the invention are set forth.

In the drawings:

Figs. 1, 2 and 3 (already referred to in the foregoing).

are diagrams, which indicate schematically the distribution of the four frequencies to the two channels in case Fig. 7 is an example of a circuit diagram showing schematically the application of the invention to the Diplex system.

In Fig. 4, relating to the Twinplex system of Fig. 1, a discriminator D is used in accordance with the invention, said discriminator being connected to the channel frequency voltage. The load resistance of this discriminator is a variable potentiometer Sp, and the discriminator applies a relatively high set of output voltages thereacross, i.e., +30 v., +10 v., -10v., or 30-,v. An amplitude clipper comprising a series resistance R and two diodes A and A being of opposite polarity serves to obtain the channel signal A, the cathode or anode of said diodes being positively or negatively biased by a voltage which can be equal to, but in this case is smaller than the direct current voltages, +10 v. and l() v., corresponding to the two frequencies nearest the central channel frequency (f and f in Fig. 1. The resistance R, is inserted in series to increase the clipping action of this amplitude clipper. The output voltage of the amplitude clipper is fed to the tube V, which is operated by means of an anode bias arrangement. The channel signal A is derived from the cathode of this tube V This tube V acts as a low impedance device. The application of an amplitude clipper to obtain the signal A is possible, because according to the diagram of Fig. 1 the direction of the shift of the instantaneous frequency from the central frequency determines the mark and space signals of the channel A. Therefore a small deviation, at most up to the frequencies f and i in Fig. 1, i.e. in the above example a direct current voltage of :05 voltage units is suificient to obtain the channel signal A. These 0.5 voltage units are produced according to Fig. 4 by bias voltage sources of +2 volts or 2 volts at the amplitude clippers A and A to obtain the proper values for the channel signal -B.

To obtain the channel signal B, the channel signal A is combined with a certain-portion of the output voltage of the discriminator. This takes place in the tube V In this operation the function explained in connection with Fig. 3 is used, according to which the total signal S at the transmitter equals the sum of the channel signals A and B, i.e.

At the receiver, the channel signal B can be derived 'by subtracting the channel signal A from the total signal S, i.e.

This subtraction is performed in the tube V by feeding to the grid the total signal S and to the cathode the channel signal A, so that the actions of these signals on the anode current are in opposite directions. In order to fulfillthe last equation, the values S and A have 'tobe brought into the correct relation to one another, which takes place by adjusting the magnitude of the total signal S by means of the potentiometer Sp. This adjustable potentiometer serves simultaneously to permit reception of transmitters having different frequency shifts by correctly adjusting the slide contact. If the slide contact is properly adjusted to the voltages indicated in Figure 4, the following equations are obtained from the last equation for the above numerical example:

cipleas the circuit of Fig. 4. However, in this ,casethe' reversing of the terminals of the one voltage is notchtaiued by controlling of the cathode of the one tube. Rather the load resistance Sp of the discriminator Dis tapped at the center. This makes it possible to superimpose the channel A and the total signal S to obtain the channel B by controlling the two tubes V and V; at the grid and operating into a. common plate resistance Ra.

In the Twinplex system shown in Fig. 6, the channel B is obtained in a dilferent manner from that in FlgS. 4 and 5. The obtaining of the channel B is based upon the fact that, for example, the positive voltages supplied from the discriminator D, said voltages corresponding to the frequencies f and f in Fig. l, are moved over into the range of the negative voltage to such an extent that the voltage corresponding to the frequency f occupies the same location as the voltage corresponding to the frequency f and that thevoltage corresponding to the frequency f occupies the same location as the voltage corresponding to the frequency f Then the space signal T appears at the output of the circuit independently of whether this signal is transmitted under the action of the channel A by the frequency f, or by the frequency i The same is true in an analogous manner for the signal condition Z. A sweep circuit K is used in Fig. 6 to select the channel signal A, or to select the channel signal B. The sweep circuit Kis simultaneously suited to obtain the channel signal A, because it has also the clipping quality of a symmetrical amplitude clipper.

The mentioned displacement for obtaining the channel signal B is carried out by means of electronic tubes V V and V V controlled by the output voltage of the discriminator D via the sweep circuit K, the outputs of said electronic tubes being interconnected and operating into a common anode resistance Ra. The inputs to these four tubes, operating as two electronic switches, are at grids of the tubes V and V These switch tubes V and V are controlled by the tubes V and V and by the circuit K in such a manner that, in case of a negative output voltage of the discriminator, the switch tube V becomes operative, and in case of a positive output voltage, the switch tube V becomes operative. This rendering operative is caused by the cut-off of the tubes V or V1- The input to the switch tube V is directly connected with the slide arm of the potentiometer Sp, while an auxiliary direct current bias fed from the voltage source Q is inserted in the electric connection between the input of the other switch tube V and the potentiometer Sp, said auxiliary direct current bias being adjusted and polarized in such a manner'that the moving-over described in the foregoing takes place. This moving-over occurs in ac cordance with the equations given for grid feed to the tube V As can be seen, the grid of the tube V is controlled by the same voltages 3 and l as the grid control of the tube V In this embodiment of this invention the receiver is adjusted to the shift separation of the four frequencies used in the transmitter by adjusting the slide contact of the potentiometer Sp or by adjusting the auxiliary direct current bias supplied by the voltage source Q.

Fig. 7- shows an application of the invention to the Diplex system according to the diagram of Fig. 2. The channel signal A is obtained by means of an amplitude clipper as in the foregoing, since the Diplex system cor-. responds to the Twinplex system with respect to the channel A. To obtain the channel signal B of Fig. 2, in the circuit of Fig. 7, the symmetrical distribution of the four frequencies representing the signals of the channel B with respect to the central frequency is utilized. The following is derived therefrom. If the output voltage of the discriminator, corresponding to the instantaneously transmitted frequency, surpasses a certain threshold value lying between the voltages of the outer frequencies, the signal T has to appear at the output of the signal channel B. The signal Z has to be located below this threshold value. Two biased diodes G and G are used to carry out this operation, said diodes becoming conductive above the mentioned threshold value and thereby permitting the output voltage of the discriminator to pass to the grids of the tubes V or V The threshold value in case of the diode G is obtained by a negative bias voltage at the anode by means of the voltage divider a, b, and in the case of the diode G by a positive bias voltage at the cathode by means of the voltage divider c, d. The grid voltages of the two tubes V and V below the threshold value given by these biasing voltages are determined by the biasing voltages on the two diodes G and G The tube V operates in accordance with the anode bias and controls the cathode of the tube V via a cathode resistance Rk common to the two tubes. The channel signal B is derived from the anode resistance Ra of the tube V If according to Fig. 2, for example, the frequency f is transmitted, said frequency produces a voltage of +3 volts at the slide arm of the potentiometer Sp, so that the diode G becomes conductive and the grid biasing voltage of the tube V increases 1 volt and the anode voltage of the tube decreases due to the correspondingly increasing anode current (space current). If the frequency f is transmitted and accordingly a voltage of 3 volts is produced at the side arm of the potentiometer Sp, the diode G becomes conductive and the grid voltage of the tube V becomes more negative by one volt. As the result of this, the cathode voltage of the tube V is decreased about one volt corresponding to an increase of the grid voltage of about +1 volt of the tube V Thus, the anode voltage of the tube V is also lowered, when the frequency f is received (space current).

Finally, a possibility for reception by the Twinplex system will be briefly explained in the following. The channel A is obtained by amplitude clipping as described in the foregoing. To lobtain the channel signal B use is made of the fact that in Fig. l the mark and space signals in the two separate channels are the same in the case of the frequencies f and f and are opposite in the case of the frequencies f and f In order to carry out this receiving method, the circuit according to Fig. 7 for the Diplex system is first employed. The output B of this circuit arrangement controls two electronic switches in such a manner that one switch is closed at the frequencies f and f and the other switch is closed at the frequencies f and f The inputs to these switches are connected to a sweep circuit controlled as in Fig, 6 by the output voltage of the discriminator in such a manner, that one conductivity condition is obtained in case of the frequencies f and f and another conductivity condition in case of the frequencies 2; and f serves simultaneously for obtaining the channel signal A. The channel signal B is derived from the common output of the two electronic switches.

Although in accordance with the provisions of the patent statutes this invention is described as embodied in a concrete form and the principle of the invention has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the elements, combinations and/or circuits shown and described are merely illustrative and that the invention is notlimited thereto, since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of the invention or from the scope of the annexed claims. The expression sweep circuit means in the foregoing specification and in claim 5 acircuit of the kind of a socalled trigger-circuit or a bistable circuit as the known flip-flop circuit of the Eccles-Jordan type.

I claim:

1. An apparatus for receiving two telegraphic signals,

both transmitted by frequency-shift mark and space transmission employing four degrees of shift symmetrically arranged about a center frequency, and separating the signals into channel A and channel B signals comprising, a descriminator to demodulate the signals and provide a total signal containing A and B components and having four different direct current voltages of polarities determined by the direction of frequency shift with respect to said center frequency; an amplitude clipper to receive output from said discriminator and eliminate voltage values in excess of the valves corresponding with the smaller shift voltages to produce a signal operative in channel A; potential divider means connected to said discriminator and adjustable to supply a signal S proportional to the discriminator output; and mixer means to receive signals .8 and A and combine these signals to produce an arithmetic resultant consti tuting the signal operative in channel B.

2. An apparatus as set forth in claim 1, the signals being related by the formula B=SA.

3. An apparatus as set forth in claim 1, said mixer means comprising an amplifier tube having an anode circuit from which the signal B is derived and having a grid circuit to which the signal S is applied and a cathode circuit to which the signal A is applied.

4. An apparatus as set forth in claim 1, said potential divider means having a grounded center and being fed at its ends by said discriminator, said clipper being fed from one end of said divider means, and in turn producing said signal A; two amplifier tubes having their anodes connected in parallel to produce the signal B, the signal A being connected to the grid of one amplifier tube, and the signal S being derived from said voltage divider means on the other side of the center from which the signal A was derived, and the signal S being connected to the grid of the other amplifier tube.

5. An apparatus as set forth in claim 1, said amplitude clipper being a sweep circuit K, the output of one side of which comprises the channel signal A, and the sweepcircuit output being control voltages of constant ampli- This sweep circuit tude with opposite polarities dependent upon the instantaneous direction of frequency shift; two amplifier tubes; and two control tubes connected to said amplifier tubes to control whether the former are conductive, said sweep circuit output voltages being connected to actuate said control tubes, and the amplifiers having their anode cir-.-

cui-ts connected in parallel to a common load resistance, across which is produced the channel signal B, the grid of one amplifier being connected to the signal S and the grid of the other amplifier being connected to the signal S through a sourch of constant direct current potential Q, which subtracts from the signal S a constant potential equalv to the absolute values of the voltage S for a maximum frequency shift frequency shift.

References Cited in the file of this patent UNITED STATES PATENTS 2,701,276 Pletscher L Feb. 1, 1955 plus the voltage S for a minimum Rudolph et a1. Apr. 6, 1954 

